$Let's\ understand\ a\ little\ with\ an\ Example:$

$.01=\dfrac{1}{2^2}=\dfrac{1}{4}=0.25$

$.11=\dfrac{3}{2^2}=\dfrac{3}{4}=0.75$

$11.01=3+\dfrac{1}{2^2}=3+\dfrac{1}{4}=3+0.25=3.25$

$101.101=5+\dfrac{5}{2^3}=5+\dfrac{5}{8}=5+0.625=5.625$

I think instead of this:

$S\to L.L \quad\{ S.val = L_1.val + L_2.val/2L_2.nb \}$

It should be this:

$S\to L.L \quad\{ S.val = L_1.val + \dfrac{L_2.val}{2^{L_2.nb}}\}$